1. Field of the Invention
The present invention relates to an optical disk on which information (e.g., digital video information) can be stored at a high density.
2. Description of the Related Art
In recent years, the recording density of optical disk media goes on increasing. On an optical disk medium on which data or information can be written by a user, a track groove has normally been formed in advance and a recording film has been formed so as to cover the track groove. Data or information is written by the user on the recording film along the track groove, i.e., either on the track groove or on an area (land) interposed between adjacent parts of the track groove.
The track groove is formed so as to wobble just like a sine wave and a clock signal is generated in accordance with a wobble period. Synchronously with this clock signal, user data is written on, or read out from, the recording film.
To write data at a predetermined location on an optical disk, address information (or location information), indicating physical locations on the optical disk, needs to be allocated to, and recorded at, respective sites on the optical disk while the disk is being manufactured. Normally, an address is allocated to a series of areas that are arranged along a track groove and have a predetermined length. There are various methods for recording such address information on an optical disk. Hereinafter, a conventional method for recording an address on an optical disk will be described.
Japanese Laid-Open Publication No. 6-309672 discloses a disk storage medium on which a wobbling track groove is discontinued locally so that an address-dedicated area is provided for the discontinued part. Pre-pits, representing address information recorded, are formed on the address-dedicated area on the track groove. This optical disk has a structure in which the address-dedicated area and a data-dedicated area (for writing information thereon) coexist on the same track groove.
Japanese Laid-Open Publication No. 5-189934 discloses an optical disk on which address information is recorded by changing the wobble frequency of a track groove. In an optical disk like this, an area on which the address information is recorded and an area on which data will be written are not separated from each other along the track.
Japanese Laid-Open Publication No. 9-326138 discloses an optical disk on which prepits are formed between adjacent parts of a track groove. These pre-pits represent the address information recorded.
These various types of optical disks have the following problems to be solved for the purpose of further increasing the recording density.
First, as for the optical disk on which address information is recorded as pre-bits within the address-dedicated area on the track, a so-called “overhead” occurs to secure the address-dedicated area, and the data area should be reduced for that purpose. As a result, the storage capacity available for the user has to be reduced.
Next, as for the optical disk for recording an address thereon by modulating the wobble frequency of the track, a write clock signal cannot be generated precisely enough. Originally, the wobble of the track groove is created mainly to generate a clock signal for establishing synchronization required for read and write operations. Where the wobble frequency is unique, a clock signal can be generated highly precisely by getting a read signal, having amplitude changing with the wobble, synchronized and multiplied by a PLL, for example. However, if the wobble frequency is not unique but has multiple frequency components, then the frequency band that the PLL can follow up should be lowered (as compared to the situation where the wobble has a unique frequency) to avoid pseudo locking of the PLL. In that case, the PLL cannot sufficiently follow up the jitter of a disk motor or a jitter resulting from the eccentricity of a disk. Thus, some jitter might remain in the resultant recording signal.
On the other hand, where the recording film formed on the optical disk is a phase-change film, for example, a signal read out from such a recording film may have a decreased SNR if data is overwritten on the film repeatedly. If the wobble frequency is unique, the noise components are removable using a bandpass filter having a narrow band. However, if the wobble frequency has been modulated, the filter should have its bandwidth broadened. As a result, the noise components are much more likely contained and the jitter might be further worsened. It is expected that the recording density will be further increased from now on. However, the higher the recording density, the narrower the allowable jitter margin will become. Accordingly, it will be more and more necessary to minimize the increase of jitter by avoiding the modulation of the wobble frequency.
In the structure in which the pre-pits representing the address information recorded are formed between adjacent parts of the groove, it is difficult to form long enough pre-pits in sufficiently large numbers. Accordingly, as the recording density is increased, detection errors might increase its number. This is because if large pre-pits are formed between adjacent parts of the groove, then those pits will affect adjacent parts of the track.